The development of the optical laser and other unique light sources has opened up a whole new area of commercial endeavor involving optical communications, optical memories, optical logic circuits, optical processing of information, etc. A particularly important class of devices associated with optical processing of information is the optical integrated circuit. Such optical integrated circuits have been described in detail in a number of references including "Integrated Optics and New Wave Phenomena in Optical Waveguides," by P. K. Tien, in Reviews of Modern Physics, Volume 49, No. 2, pp. 361-381, April 1977, and "Guided Wave Optics," by H. F. Taylor and A. Yariv, Proceedings of the IEEE, Volume 62, No. 8, pp. 1044-1060, August 1974. A recent review of this subject matter is given by R. C. Alferness in a paper entitled, "Guided-Wave Devices for Optical Communication," IEEE Journal of Quantum Electronics, Volume QE-17, No. 6, June 1981.
A particular procedure for fabricating guided-wave devices involves use of a lithium niobate substrate. Adjustment of the composition of the lithium niobate is particularly important so as to form a low-loss homogeneous medium for the optical device. Indeed, a homogeneous substrate material is required to provide the uniform index of refraction needed for good integrated device performance. A rapid, inexpensive, reliable procedure is needed for the commercial manufacture of lithium niobate substrates with a specific homogeneous composition and uniform optical properties. A particular procedure for adjusting the composition of lithium niobate is described in U.S. Pat. No. 4,071,323 issued to R. L. Holman on Jan. 31, 1978.
In addition to adjusting the composition of the lithium niobate and homogenizing the composition and optical properties of the lithium niobate substrates, it is highly desirable to have a simple, convenient procedure for maintaining composition and homogeneity during fabrication procedures involving heating. For example, diffusion of various elements into lithium niobate substrates to form waveguide sections often is accompanied by loss of homogeneity or composition change in the lithium niobate substrate.